1. Field of the Invention
The present invention relates to a process for the preparation of a discoloration-free, high-molecular weight (co)polycarbonate by polycondensing a dihydroxy compound with a carbonic diester in the presence of a transesterification catalyst.
2. Description of the Related Art
A high-molecular weight polycarbonate is a general-purpose engineering thermoplastic useful in various fields, particularly as a material for injection molding or a sheet substituting for a window pane. Polycarbonates are generally believed to be excellent in heat resistance, transparency and impact resistance.
Ordinary processes for the preparation of a polycarbonate include the phosgene process wherein the interfacial polycondensation of a dihydroxy compound with phosgene is conducted and the transesterification process wherein a dihydroxy compound is reacted with a carbonic diester in a molten state.
A representative example of the transesterification process comprises reacting a dihydric phenol with a carbonic diester in the presence of a transesterification catalyst and during heating under a reduced pressure while distilling off formed phenol to form a prepolymer and converting the prepolymer into a high-molecular weight polycarbonate by bringing the resulting system finally to 290.degree. C. or above in a high vacuum while distilling off formed phenol [see U.S. Pat. No. 4,345,062 (date of patent: Aug. 17, 1982; assignee: GENERAL ELECTRIC CO.)].
In the transesterification process, it is known that a process which comprises forming a prepolymer in an ordinary tub-type reactor fitted with an agitating blade in the early stage of the reaction and thereafter polycondensing the resulting reaction mixture comprising the prepolymer in a horizontal vented extruder or the like is effective in advancing the reaction efficiently to form a high-molecular weight polycarbonate.
Unlike other engineering plastics, a high-molecular weight polycarbonate, however, has such a high melt viscosity that the preparation thereof necessitates a reaction temperature of as high as 280.degree. C. or above. Further, a high vacuum (of 1 to 10.sup.-2 Torr) is also necessary for distilling off a formed high-boiling monohydric compound. Accordingly, the industrial production of a high-molecular weight polycarbonate is difficult from the standpoint of plant facilities and, further, there is a problem that the reaction conditions described above have a bad influence upon the qualities of the formed polycarbonate, such as hue, heat resistance, residence stability in molding, water resistance, weathering resistance and so forth.
It has been known that the material of a reactor has an influence on the discoloration of the resin prepared in the reactor. For example, it is described in Japanese Patent Publication-A No. 55-142025 and other patent documents that when a resin is prepared in a reactor having a part in contact with the reaction liquid of stainless steel, the resin is discolored. It is described in Japanese Patent Publication-A No. 2-153923 (published on Jun. 13, 1990) that it is preferable for eliminating the influence of a reactor material on the hue of the resin that a least 90% of the total area of the surface in contact with the reaction liquid, i.e., liquid-contacting surface, comprises one or more members selected from among glass, nickel, tantalum, chromium and Teflon. Further, it is described in Japanese Patent Publication-A No. 4-72327 (published on Mar. 6, 1992) that it is preferable that the material of the liquid-contacting surface be a metal or metal alloy containing copper and/or nickel in an amount of 85% by weight or above. Furthermore, the present inventors have disclosed the usefulness of a material having an iron content of 20% by weight or below for the reactor in Japanese Patent Publication-A No. 4-88017 (published on Mar. 19, 1992). Although these means are effective in inhibiting the discoloration of a resin, they have problems in that the use of an expensive material is substantially unavoidable and that the application of a special material to a device having a complicated shape is difficult from the standpoints of processability and obtainment thereof in many cases, which brings about an increase in the cost of equipment, leading directly to an increase in the cost of a product finally.
It has also been known that a boron compound such as boric acid and triphenylborate is useful to prevent the discoloration of the polycarbonate [see U.S. Pat. Nos. 3442854 (assignee: FARBENFAB BAYER AG), 4,590,257 (assignee: GENERAL ELECTRIC CO.; date of patent: May 20, 1986) and 5276109 (assignee: GENERAL ELECTRIC CO.; date of patent: Jan. 4, 1994), and European Patent Publication-A Nos. 351168 (published on Jan. 17, 1990) and 435124 (published on Jul. 3, 1991)]. However, the use of the boron compound sometimes brings about other problem(s).
Under the circumstances described above, the development of a process for the preparation of a polycarbonate at a low cost has eagerly been expected.